The basic building block for a large class of antennas is the half-wave dipole. This consists of a length of wire or tubing, generally fed at the center, which resonates at a frequency corresponding to a wavelength of twice the length of the dipole. For many applications, the physical length of a half-wave dipole is too great to be accommodated in the available space, and a great deal of research effort has been expended in finding ways of reducing antenna size without compromising performance. There are many techniques currently in use to reduce the size of an antenna element. The basic problems with short elements are:
A short element has capacitive reactance that must be ‘tuned out’ in order for the element to accept power. The inductance required can be produced by a conventional solenoid inductor or by using a short-circuited transmission line, or by other means, but all of these methods reduce antenna efficiency because of ohm losses in the inductor, however made.
The bandwidth of a reduced size antenna is substantially reduced from that of a full size half-wave dipole. This means that it can be impossible to cover a desired frequency range without re-tuning the antenna
The radiation resistance of the reduced size antenna is substantially lowered. This low resistance must be matched to the antenna feed line in order to avoid high standing wave ratio, with associated power loss, in the feed cable. Matching circuits can be used but they also have associated power loss. Techniques for feeding the antenna off-center are commonly used to raise the input resistance, but have problems associated with a feedpoint that is not located at a voltage node. Alternative matching solutions include the “gamma match” and the “T match” and other similar systems, but they all have in common difficulty in combining several short elements in order to obtain operation over many different frequency bands.
Other small antennas that are commonly in use include the magnetic loop antenna, characterized by very low radiation resistance, low bandwidth and a need to be remotely tunable in order to provide multi-band coverage.
The present antenna element disclosed herein has reasonable bandwidth, a low loss impedance transformation capability built in to its structure to allow direct connection to a feed cable, and the ability to be connected to other reduced size elements in order to provide multi-band operation without using switching or matching circuits. It may also be used in high directivity yagi-like antennas.
There is a plethora of information in the prior art relating to the design and use of physically small antennas. Some basic limits to the bandwidth and Q factor associated with small antennas are developed in Richard C. Johnson, “Antenna Engineering Handbook.” (Third Edition, McGraw-Hill, Inc., New York) Small antennas and their limitations are discussed in John D. Kraus, “Antennas,” (Second Edition, McGraw-Hill, Inc, New York, 1988) and in John A. Kuecken, “Antennas and Transmission Lines,” (First Edition, Howard W. Sams & Co. Inc, New York, 1969). Design problems and solutions for short antennas principally for use in hand-held radio communication devices are discussed in K. Fujimoto et al., “Small Antennas,” (John Wiley and Sons, Inc., New York 1987). U.S. Pat. No. 3,083,364 (to Scheldorf) discloses a helical monopole antenna of reduced physical size, designed to be used in conjunction with a ground plane, that incorporates a structure similar to that of a folded dipole that increases the feedpoint impedance such that, for example, a coaxial cable having a characteristic impedance of 50 ohms can be directly connected thereto.
Prior art solutions to resonating a physically small dipole antenna so that it will accept power from a transmission line all use inductive loading techniques, and may use capacitive end loading techniques in order to increase radiation resistance and lower the amount of loading inductance needed. FIG. 1 shows a short dipole (also known as a “Hertzian” dipole), consisting of an element much shorter than a half-wavelength (2), and a source (1). As an example, if the length of the dipole shown in FIG. 1 is 0.15λ and the dipole is fabricated from wire of diameter 0.001λ, the radiation resistance is only approximately 4.4 ohms and the series reactance is −j930 ohms.
In order to feed power to the above antenna, the feedpoint resistance has to be transformed up to 50 ohms, and the capacitive reactance must be tuned out with a series inductance, or circuit equivalent thereto, having a reactance of +j930 ohms. Such an antenna system is illustrated in FIG. 2. A loading inductor 3 is inserted in series with the element 2 and the source 1. Prior art also uses capacitive end-loading of the dipole in order to reduce the capacitive reactance. Such end-loading may consist of discs or skeleton discs connected to each end of the dipole, such discs being of diameter substantially larger than that of the wire. FIG. 3 shows such an antenna. The end capacitive loading is provided by wires 4 and 5, and the inductor 3 in series with the element 2 and the source 1 may be smaller and less lossy than that needed when no end loading is used. Other loads used in the prior art consist of a length of wire running at right angles to the dipole wire axis and parallel to a ground-plane, as in an inverted L antenna illustrated in FIG. 4, where the source 1 drives the element 2 over a ground-plane 6. When such end-loads are used, not only is the capacitive reactance reduced, but also the radiation resistance is increased. The maximum value of radiation resistance that can be achieved by end-loading a short dipole is four times that of the unloaded dipole. For an inverted L antenna, as in FIG. 4, and the total wire length is approximately 0.25λ, then the antenna is resonant and the reactance of the short dipole is completely cancelled out. Thus the feed point impedance is purely resistive and all that is needed in order to feed the antenna with power in a 50 ohm system is a transformer, or to tap the source along the antenna at a point that shows a 50 ohm load resistance.
Although there are many different wire configurations found in the prior art, they are all basically similar in that they attempt to raise the radiation resistance and cancel, or minimize, the series capacitive reactance.
The prior art in directive antennas, such as yagis, is well covered in R. Dean Straw et. al., “The ARRL Antenna Handbook,” (19th Edition, American Radio Relay League, Newington, 2000). Single band yagis normally consist of an array of elements that are approximately 0.5λ long spaced from each along a boom, as shown in FIG. 5. A source, 1, drives the driven element, 2. The reflector, 7, and the directors, 8, 9, and 10 (more or less directors may be used than illustrated) are coupled by the electromagnetic field in such a way that the antenna becomes directional, with the direction of the beam being along the boom away from the reflector. The directivity or gain of such an antenna having many directors, know as a long yagi, is given approximately by:Gain G≈10 log10 10Lλ, where Lλ is the boom length in wavelengths.Multiband operation is commonly achieved by placing parallel resonant circuits know as “traps” in pairs in series with each of the elements: these traps effectively isolate the outer sections of the elements and allow resonance and antenna operation at a second, higher frequency. However the traps are lossy and cause power and gain loss, and also reduce the operating bandwidth. They are rarely used in long yagis that operate in the vhf bands and above because of the number of traps needed, thus almost all high performance long yagis for vhf and upwards are single band devices.